1. Field of the Invention
The present invention is directed to technology and design of efficient ultrasound transducers for high frequencies, and also transducers with multiple electric ports for efficient operation in multiple frequency bands, for example frequency bands with a harmonic relation. The invention has special advantages where the highest frequencies are above 10 MHz, but has also applications for transducers at lower frequencies.
2. Description of the Related Art
Medical ultrasound imaging at frequencies above xcx9c10 MHz, has a wide range of applications for studying microstructures in soft tissues, such as the composition of small tumors or a vessel wall. In many of these situations it is also desirable to use ultrasound pulses with frequencies in several frequency bands, for example to
1. use a pulse with frequencies in a low frequency band of for example 30-40 MHz to get larger image depth for an overview image, and then be able to switch to or use simultaneously a pulse with frequencies in a high frequency band, say 60-80 MHz, for high resolution imaging of close structures in a shorter depth of the image, or
2. to transmit an ultrasound pulse with frequencies in a low frequency band, say around 30 MHz, and receive back scattered signal components at a harmonic of the transmit band, say a 2nd harmonic band around 60 MHz, a 3rd harmonic band around 90 MHz, or even a sub-harmonic band around 15 MHz.
Ultrasound transducers for medical applications are currently based on ferro-electric, ceramic plates as the active material, that vibrates in thickness mode. When polarized, the materials show piezoelectric properties with efficient electromechanical coupling. However, the characteristic impedance of the ceramic material (Zxxcx9c33 MRayl) is much higher than that of the tissue load material (ZLxcx9c1.5 Rayl). In order to get adequate thickness vibration amplitude of the plate for efficient power coupling into the tissue load material, one must operate the plates at thickness resonance, typically Lx=xcex/2 resonance. Here Lx is the plate thickness, xcex=c1/f is the wavelength of longitudinal waves normal to the plate with wave velocity c1 and frequency f. The resonance makes the transducer efficient in a band of frequencies around a center frequency f0=c1/xcex0=c1/2Lx. Acoustic matching plates between the ceramic plate and the load are used to improve the power coupling to the load, a technique that increases the bandwidth of the transducer resonance.
With the well known composite technique, where the ceramic plate is diced into small posts, and the interpost space is filled with epoxy, the efficient characteristic impedance is reduced to xcx9c15 MRayl, which is still around 10 times higher than the characteristic impedance of the load, such as soft tissue or water. Transducers of composite material must therefore also operate in thickness resonant mode, albeit one can obtain some wider bandwidth than with the transducers of whole ceramic.
Hence, both with whole and composite ceramic, the resonant operation requires that the plate thickness is inversely proportional to the center frequency of the operating transducer band.
This requires thicknesses in the range of 200-20 xcexcm for center frequencies in the range of 10-100 MHz. Today, lapping of the ceramic plate is the common technology to manufacture plates with correct thickness, which becomes difficult and expensive at thicknesses in ranges below 50-60 xcexcm, corresponding to frequencies above 30-40 MHz. Composite ceramic/epoxy material is also difficult to make for frequencies above 15 MHz, and it is hence a general need for efficient methods to manufacture transducers with a functioning high frequency band above 15 MHz.
The invention presents a new design of ultrasound transducers where the active electromechanical coupling material is ferroelectric, ceramic films that are made piezoelectric through electric polarization. The piezoelectric film layers are arranged into a transducer plate composed of multiple film layers, possibly also non-piezoelectric layers, where all the film layers have close to the same characteristic impedance. For coupling of the vibrations to an electric port, electrodes are placed inside the plate structure with a piezoelectric layer between the electrodes to form an electric port of the transducer, which interacts with the acoustic port of the transducer plate surface. By placing the electrodes inside the plate, the distance between the electrodes can be made substantially shorter than the total thickness of the transducer plate, which is an important aspect for high frequency operation of the transducer according to the invention.
The electromechanical coupling of the electrode port is highest at the frequencies where the thickness vibrations of the piezoelectric layer between the electrodes, is maximum. Due to reflections inside the transducer, one obtains a standing wave vibration pattern within the plate. The maximal vibration amplitude in the plate is found at the plate resonances, and to transform the resonant vibration amplitude to a large thickness vibration of the material between the electrodes, one must also place the electrodes at antinodes with opposite vibration direction in the standing wave vibration pattern. This gives a distance between the electrodes xcx9cxcex/2, where xcex is the wave length in the material. Hence, the highest sensitivity of the transducer is found when the transducer plate is at a thickness resonance and the distance between the electrodes is xcx9cxcex/2 with correct placement at antinodes in the standing wave pattern. With very high backing impedance, the back interface is a node, and it can pay to put one electrode at the back interface and the other at the antinode at xcex/4 distance in front of the back electrode.
The close to constant characteristic impedance within the transducer plate implies that the mechanical thickness resonances of the transducer are determined by the total plate thickness, not by the thicknesses of the individual film layers that composes the plate. By placing electrodes inside the transducer plate, the transducer plate can operate over a larger range of resonances, from xcex/2 to multiple xcex resonances, while the electrodes at the center frequency are placed at antinodes with distance xcx9cxcex/2 internal in the transducer plate, maximizing the electromechanical coupling of the electrodes over the actual frequency band. This allows the use of thicker transducer plates than the standard xcex/2 transducer plates, which provides manufacturing advantages as described below.
The plate is hence so much thicker than the active material between the closest electrode layers, that the phase angle of the wave propagation through the film layers outside these active layers has a substantial, non-negligible effect on the mechanical thickness resonances of the whole plate. We shall say that a layer has a thickness substantially larger than another layer when the difference between the two layers of the wave propagation phase angle is non-negligible in the determination of resonances. Similarly, a layer has non-negligible thickness when the propagation phase angle is non-negligible in the determination of resonances
Film layers outside the active ceramic material can be made of other types of material with similar characteristic impedance as the ferroelectric, ceramic film, for example layers of conductive film. Conducting layers can have a combined function as electrodes, and as vibrating layer with non-zero thickness for the definition of the transducer plate thickness resonances. One simple design of the transducer according to the invention, is an active ferroelectric ceramic layer with a thin electrode with negligible propagation phase angle on the back side, and a conducting layer on the front side which both functions as a front electrode and an elastic layer that makes the total plate substantially thicker than the active piezoelectric layer. Such a conducting layer can for example be made as a film of an Ag/Pd mixture. Other examples with more than two electrodes that gives multiple electric ports for multi-band operation of the transducer, is shown in the specification below.
The multi layer structure can be made with tape casting of the films, or deposition onto a substrate with thick film printing, sol-gel deposition, or other deposition techniques. With tape-casting techniques one can typically make films with thickness in the range of xcx9c10-30 xcexcm. The raw films before sintering are quite pliable, and layers of films can be stacked to form plates of larger thickness. The films are sintered at temperatures xcx9c1000xc2x0 C., which makes the plate brittle and limits the lower thickness of self-supporting plates and hence the highest operable frequency with ordinary xcex/2 resonant transducer plates made with tape casting techniques. By placing electrodes inside the plate as described, one can obtain efficient electro-acoustic coupling at frequencies where the total plate thickness Lx is substantially larger than xcex/2, allowing increased thicknesses Lx of the total transducer plate that increases the stability during the sintering process and other handling of the plate. The design is specially useful for operating frequencies above xcx9c30 MHz.
With deposition of the ceramic films onto a substrate, one has a problem that many actual substrate materials contaminate the ferroelectric ceramic film during the sintering processes, so that in the neighborhood of the substrate, the film loses its ferroelectric properties, and hence also its piezoelectric properties. Substrates that withstand the sintering process without destruction of the ceramics ferroelectric properties, are rigid so that they produce ringing in the transducer vibrations after the pulse transmission. The invention devises a solution to the contamination problem by using a non-piezoelectric isolation layer between the substrate and the active, piezoelectric, electromechanical coupling layers, with characteristic impedance close to that of the piezoelectric layer. This layer can be made of a ceramic film that is allowed to be contaminated during the sintering process without reducing the transducer function, or other materials, like for example zirconium (Zr) or mixtures of silver (Ag) and palladium (Pd). One can then use substrates, like silicon (Si), that can be etched after the sintering process to such low thickness that the ultrasound can be transmitted through the remnant substrate layer that functions as a load matching layer.
Load matching layers are used for acoustic connection between the transducer plate and the load material to increase the bandwidth of the mechanical resonances of the plate. Manufacturing of load matching layers with correct thickness and characteristic impedance at these high frequencies (i.e. thin layers) presents problems. The invention devices a solution to these problems by prescribing materials that can be adjusted to the correct thickness by electroplating or etching. Layers with adjustable characteristic impedance can be made as a composite of solid and polymer materials by etching grooves in the solid material, and filling the grooves with polymer, or in some situations the grooves can be unfilled. Alternatively one can grow posts or other structures of solid material by electroplating onto a substrate, the dimensions being controlled by photo-lithographic techniques. Using such techniques to form a casting frame for ceramics, one can also make ceramic/polymer composite films, or a ceramic post matrix where the inter-post volume is unfilled, to reduce the characteristic impedance of piezoelectric films.
By introducing more electrodes both at the front face of the transducer plate and between film layers inside the composite plate, one obtains multiple electric ports that can be efficient in different frequency bands. Electrodes at both faces of the transducer plate for example, can be used as a lower frequency electric port that is efficient around lower resonance frequencies of the plate, for example xcex0/2 resonance at f0=c1/2Lx for the low characteristic impedance backing. According to the invention, the signals from several electric ports can be combined for improved transmit and receive characteristics, either through direct galvanic connection of electrodes, or in transmit mode through special drive signals on the electrodes, or in receive mode through a combination of the signals from electric ports after isolation amplifiers.
The structure can be laterally divided into array elements to obtain transducer arrays for electronic direction steering and focusing of the ultrasound beam.
Other objects and features of the present invention will become apparent from the following detailed description considered in conjunction with the accompanying drawings. It is to be understood, however, that the drawings are designed solely for purposes of illustration and not as a definition of the limits of the invention, for which reference should be made to the appended claims. It should be further understood that the drawings are not necessarily drawn to scale and that, unless otherwise indicated, they are merely intended to conceptually illustrate the structures and procedures described herein.